This invention relates to vacuum pumping systems and, more particularly, to such systems for use in controlling the pressure in a semiconductor processing chamber.
The requirements for a vacuum pumping system for use in the semiconductor industry are many and varied. In addition to evacuating the semiconductor processing chamber down to the required level of vacuum and exhausting the reaction gases used in the chamber in the manufacture of semiconductor devices from the chamber to atmosphere or to one many types of collection or scrubbing means, the pumping system is increasingly being used to control the pressures associated with the processing chamber by varying the rate at which the reaction gases are exhausted from the chamber.
In particular, there is a need in the semiconductor industry to provide a control on the pressure in the processing chamber independently of the process reaction gas flow quantity in, and from, the chamber. In addition, there may also be a need to provide a control on the reaction or other gas species present in the processing chamber in order to vary partial pressures of the reactive gases and reactive gas by-products, for example to exhaust the reactive by-products from the chamber at a rate faster than that of the reactive gases themselves or to promote means to reduce the time between cleaning operations in a chamber and the normal processing operations.
In a typical simple vacuum pumping system for use in the semiconductor industry, the processing chamber is connected to a system comprising a first vacuum pump (or pumps)--commonly a turbo-molecular pump--which is backed by a forepump (or pumps) connected to the first pump by a foreline and which can exhaust the gases from the semiconductor chamber to atmosphere.
In such a simple system, in an attempt to provide a means to exercise control on the pressure in the processing chamber to which it is attached, it has previously been prepared to provide a variable throttle valve either between the process chamber and the first pump(s) or in the foreline between the first pump(s) and the fore pump(s).
However, it has been found that the presence of a throttle valve can cause certain disadvantages. For example, if the throttle valve is at the inlet to the first pump, there is necessarily a restriction in to that pump even when the throttle valve is fully open, so that a larger and therefore more costly first pump (or pumps) is required.
If the throttle valve is in the foreline, the effect of it on the pumping rate of the first pump is to render it highly non-linear so that it becomes effective only over a narrow range of pressure. As such, the system as a whole is difficult to regulate in a stable manner if the process gas flow rate varies by a large amount.
In such a simple pumping system, it has also been proposed to introduce a variable flow of a ballast gas (or a spoiling gas) in to the foreline. However, this has generally not proved effective in allowing a control of the pressure in the processing chamber. In addition, the introduction of a ballast gas of a different composition to that used in the semiconductor processing may contaminate or dilute the process gases. If it is of the same composition, the flow rate may be large and therefore costly.
Furthermore, for the same general reasons, it has also been proposed to provide means to regulate the rotational speed of the first pump(s) or the forepump(s) or both. However, regulation of the rotational speed of the first pump, for example a turbo-molecular pump, cannot normally be achieved rapidly without requiring a large amount of extra power, due to the large moment of inertia of the pump rotor. This leads to the need for more expensive motor and drive electronics. Alternatively, the time required to regulate pressure in the processing chamber is long which in itself reduces the effectiveness of the pumping system as a whole.
Regulation of the rotational speed of the forepump suffers from the same disadvantages and, additionally, can make the pumping rate of the first pump non-linear and effective only over a narrow region of pressure.
Attempts to provide means to regulate the rotational speed of both the first pump and the forepump result in further expense and complexity and, in any event, do not fully overcome the disadvantages.
There is therefore a need for alternative means in such pumping systems for controlling the pressure as necessary in the processing chamber.